feat(sprints): modèle de sprints — domaine, use-cases, persistance et surfaces (backend)

Ticket #10 — introduction du modèle de sprints côté backend.

- Domaine : nouvel agrégat Sprint (sprint.rs), IDs, événements et invariants ;
  rattachement des issues à un sprint (issue.rs) et ports associés.
- Application : use-cases sprints (application/src/sprints) + erreurs dédiées.
- Infrastructure : store de sprints (infrastructure/src/sprints.rs), adaptation
  du store d'issues et exposition MCP via orchestrator/mcp/tickets.rs.
- app-tauri : commandes, state et events pour piloter les sprints depuis l'UI.

Tests domaine/application/infra/app-tauri verts (sprint_usecases, sprint_store,
issue_store, mcp_server).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/tests/issue_store.rs

@@ -4,7 +4,7 @@ use std::str::FromStr;
 use domain::{
     AgentIssueRef, AgentIssueRole, Issue, IssueActor, IssueId, IssueListFilter,
     IssueNumberAllocator, IssuePriority, IssueRef, IssueStatus, IssueStore, IssueStoreError,
-    MarkdownDoc, ProjectPath,
+    MarkdownDoc, ProjectPath, SprintId,
 };
 use infrastructure::{FsIssueNumberAllocator, FsIssueStore};
 use uuid::Uuid;
@@ -144,6 +144,44 @@ async fn issue_store_lists_by_index_filters() {
     assert_eq!(rows[0].title, "Beta");
 }
 
+#[tokio::test]
+async fn issue_store_persists_and_filters_sprint_membership() {
+    let tmp = TempDir::new();
+    let root = tmp.root();
+    let store = FsIssueStore::new();
+    let sprint_id = SprintId::from_uuid(Uuid::from_u128(42));
+    let assigned = issue(&root, 1, "Assigned")
+        .mutate(IssueActor::System, 2_000, |i| {
+            i.sprint = Some(sprint_id);
+        })
+        .unwrap();
+    store.create(&root, &assigned).await.unwrap();
+    store
+        .create(&root, &issue(&root, 2, "Backlog"))
+        .await
+        .unwrap();
+
+    let loaded = store
+        .get_by_ref(&root, IssueRef::from_str("#1").unwrap())
+        .await
+        .unwrap();
+    assert_eq!(loaded.sprint, Some(sprint_id));
+
+    let rows = store
+        .list(
+            &root,
+            IssueListFilter {
+                sprint: Some(sprint_id),
+                ..IssueListFilter::default()
+            },
+        )
+        .await
+        .unwrap();
+    assert_eq!(rows.len(), 1);
+    assert_eq!(rows[0].title, "Assigned");
+    assert_eq!(rows[0].sprint, Some(sprint_id));
+}
+
 #[tokio::test]
 async fn allocator_never_reuses_numbers() {
     let tmp = TempDir::new();